In recent years, structures configured with a plurality of steel sheet members made out of steel sheets are commonly configured to include an overlap-welded member having the plurality of steel sheet members joined with each other in a manner such that the steel sheet members are overlapped with each other in a manner according to the functions thereof or the environments in which they are used, to make an overlapped portion, and this overlapped portion is subjected to resistance spot welding to create a spot-welded portion having a nugget.
For example, a monocoque body (automotive part), which constitutes an automobile body, is generally formed by overlapping steel sheet members including high-strength steel sheets, and applying resistance spot welding to a flange portion (overlapped portion) in order to achieve both an improvement in collision safety and an improvement in fuel efficiency.
Currently, high-tensile steel sheets having a tensile strength of 980 MPa class are widely used as high-strength steel sheets for automobiles, and further, investigations have been performed on high-tensile steel sheets having a tensile strength of 1200 MPa class or higher.
Furthermore, another technique has been under investigation, which employs hot stamping in which both press forming and quenching are performed simultaneously in the same die to form the steel sheet members of the overlap-welded member, and manufactures hot stamps having a tensile strength of 1500 MPa or higher as the steel sheet members.
During this hot stamping, the steel sheets are heated to high temperatures to be in a highly ductile state, and then press forming is performed. Thus, high-strength steel sheet members having a tensile strength of 1500 MPa or higher can be efficiently manufactured, and further, the dimensional accuracy after the press forming can be advantageously improved.
For example, in the case of structures required to have a rust resistance, these structures may be made by overlapping steel sheet members made out of steel sheets including galvanized steel sheet having a hot-dip galvanneal coating or a hot-dip galvanizing coating formed thereon, and then joining the overlapped portion through resistance spot welding.
For example, an outer panel, constituting the monocoque body, generally includes galvanized steel sheets having a hot-dip galvanneal coating or a hot-dip galvanizing coating formed thereon.
The steel sheet members formed by applying hot stamping to high-tensile steel sheets or high-strength steel sheets having a tensile strength of 1200 MPa or higher, as described above, usually contain a quenched structure.
However, the spot-welded portion, at which the overlapped portion is joined, has a heat-affected zone (hereinafter, referred to as HAZ) of which hardness is less than the base material containing the quenched structure, because heat from the resistance spot welding causes tempering of the quenched structure around the nugget.
This softening of the HAZ which has a hardness lower than that of the base metal may also occur in the case of steel sheets of 980 MPa class. However, this softening occurs particularly in the spot-welded portion of a high-tensile steel sheet having a tensile strength of 1200 MPa or higher and containing quenched structures formed with continuous annealing equipment having a water cooling function, or a hot stamped member (high-strength steel sheet member) formed through hot stamping.
For example, in the case of a cold-rolled steel sheet of 1200 MPa class, the base metal has a Vickers hardness of approximately 390, whereas the softest zone in HAZ has a Vickers hardness of approximately 300.
This means that Vickers hardness is approximately 90 less than the base metal.
FIG. 14 is a diagram showing an example of a test piece 100 including an overlapped portion obtained by overlapping a hot stamped member (high-strength steel sheet member) 101P having a tensile strength of 1500 MPa class and formed through hot stamping and a hot stamped member (high-strength steel sheet member) 102P having a tensile strength of 1500 MPa class and formed through hot stamping, and forming a spot-welded portion 110 having a nugget 112 through resistance spot welding, and further showing a distribution of hardness according to Vickers hardness (JIS Z2244) measured by applying indentations to positions located along the broken line shown in the schematic view showing the test piece 100 (positions located at one-quarter of the thickness (thickness/4) from the interface in a direction towards the center of the steel sheet).
The Vickers hardness is measured with a load of 9.8 N and at pitches of 0.5 mm.
As shown in the graph in FIG. 14, the test piece 100 is formed by overlapping the hot stamped member 101P of 1500 MPa class formed through hot stamping and the hot stamped member 102P of 1500 MPa class formed through hot stamping. This test piece 100 exhibits a Vickers hardness of approximately 450 at the base material (hot stamped member 101P) and a Vickers hardness of approximately 300 at the softest zone 103A in the HAZ softening zone 103.
In other words, Vickers hardness at the softest zone 103A in the HAZ 103 decreases by approximately 150 in comparison with that at the base metal (hot stamped member 101P).
Then, tensile load is applied to the test piece 100. As a result, a fracture occurred from the HAZ softening zone 103 located outside of and in the vicinity of the nugget 112 as shown in FIG. 15A and FIG. 15B.
FIG. 15A is a sectional view of the surface of the steel, and shows how the hot stamped member 101P of the test piece 100 shown in FIG. 14 fractured. FIG. 15B is a sectional view showing the state of the fracture from the HAZ softening zone 103.
The softening of HAZ, as described above, does not influence the evaluation results of tensile shear tests and cross tension tests (JIS Z3137) used for joint evaluation of resistance spot welding. However, in the case where a tensile load is applied to the test piece 100 as shown in FIG. 15A and FIG. 15B, distortion is focused locally on the HAZ softening zone 103, possibly causing the fracture in the HAZ softening zone 103.
The above-described fracture in the HAZ softening zone of the spot-welded portion can be seen in the steel sheet member (press formed article) formed by steel sheets having a tensile strength of 1200 MPa or higher, and there is a possibility that the advantage of the high-strength steel sheet cannot be fully achieved at the time of impact.
For example, structural elements (overlap-welded members) such as an A-pillar, a B-pillar, a roof rail, and a side sill constituting the automobile body are required to protect occupants in a cabin at the time the automobile collides.
For this reason, deformation at the time of impact is suppressed by overlapping a plurality of steel sheet members, and joining the flange (overlapped portion) through resistance spot welding, thereby forming a tubular closed cross section.
However, in the case of serious impact modes such as SUV side crash tests of the Insurance Institute for Highway Safety (IIHS) and pole side impact tests of the Euro NCAP, there is a possibility that it is difficult to achieve predetermined impact performance even using, for example, high-strength steel sheets, because deformations concentrate on the HAZ softening zone of the spot-welded portion and the HAZ softening zone serves as a starting point of fracture.
Thus, in order to make full use of the performance of the high-strength steel sheet, it is necessary to prevent the HAZ softening zone of the spot-welded portion from serving as the starting point of fracture in the case where the structural elements of the automobile body include the steel sheet member formed by the high-strength steel sheet having a tensile strength of 1200 MPa or higher.
For example, one of the techniques disclosed includes a technique of alleviating stress at the welded portion and suppressing delayed fracture by employing both laser welding and spot welding in the case where high-strength steel sheets are welded to form a structural element for automobiles (see, for example, Patent Document 1).
Furthermore, in connection with improvement in joining at a welded portion, a technique is disclosed in which continual welded portions are formed along the spot-welded portion of a metal material through laser welding (see, for example, Patent Document 2).
Furthermore, in connection with improvement in joining at a welded portion, a technique is disclosed in which laser light is emitted onto a spot-welded portion or the vicinity of the spot-welded portion to laser weld a steel sheet on the surface side and a thick steel sheet adjacent to this steel sheet on the surface side (see, for example, Patent Document 3).
Furthermore, Non-Patent Document 1 discloses a method of preventing structural elements from fracturing from a HAZ softening zone, in which the strength of a base metal is reduced through thermal treatments applied at the time of hot stamping to a portion having a risk of causing a fracture due to impact to an A-pillar formed through hot stamping, whereby the softening of HAZ does not occur even if resistance spot welding is applied.
Furthermore, Non-Patent Document 2 discloses a method of preventing structural elements from fracturing from a HAZ softening zone, by reducing the strength of a base metal through tempering using high frequency heating applied to a flange portion of a B-pillar formed through hot stamping, thereby preventing the HAZ from softening even if resistance spot welding is applied.